Electric power steering device

ABSTRACT

An electric power steering unit comprises a housing (A) for containing a rack-shaft and holding therein a coupling section of the rack-shaft to a pinion of a steering column and an electric motor equipped with a yoke ( 7 ) arranged inside the housing (A) and integrally formed therewith, and a ball screw mechanism ( 3 ) contained in a housing (B) for transmitting the rotating force of the motor to the rack-shaft as the steering assisting power. A female threaded portion ( 36 ) is formed at the end portion of yoke ( 7 ) on the housing (B) side. The female threaded portion ( 36 ) is fitted to a male threaded portion ( 37 ) formed on the end portion of housing (A) on the housing (B) side to couple the housing (A) to the housing (B). As a result, wear and adhesion are prevented as well as wall thickness is reduced with the coupling strength of the housings maintained in the electric power steering unit with a configuration in which the housings are coupled with screws.

TECHNICAL FIELD

This invention relates to an electric power steering unit to be used fora vehicle and, more particularly, it relates to a field of technologyeffectively applicable to electric power steering gears to be used forrack and pinion type steering systems.

BACKGROUND ART

For assisting steering power of vehicles, a so-called power steeringunit is equipped to many vehicles in recent years, and various powersteering units such as hydraulic type, electric type, etc. are proposed.Of these power steering units, for the electric type power steering unitapplied to rack and pinion type steering units (so-called electric powersteering), the one called rack-assist type that obtains steeringassisting power by a motor equipped close to the rack shaft is known.

FIG. 5 is an explanatory view showing one example of this kind ofrack-assist type power steering unit. As shown in FIG. 5, the electricpower steering unit has a motor 52 installed coaxially with a rack shaft51, and the steering assisting power generated by the motor 52 istransmitted to the rack shaft 51 via a ball screw mechanism 53. And withthis steering assisting power and manual steering power, guiding wheelsare turned to alleviate steering loads of a driver.

The rack-shaft 51 is linked to the guiding wheels typically by way oftie rods or knuckle arms arranged at the respective opposite endsthereof and also linked to the steering column 54 that is coupled to thesteering wheel by way of a rack and pinion gear so that it may bereciprocatively moved in the horizontal directions of FIG. 5 as thesteering operation by the driver. The motor 52 is arranged in a housing71 containing coupling section 68 between the steering column 54 and therack shaft 51, and a cylindrical armature shaft 56 and a field device 57coaxially inserted in a cylindrical yoke 55. In the power steering unitof FIG. 5, the yoke 55 is formed integral in the housing 71. Inaddition, the field device 57 comprises a magnet 59 installed to theinner circumferential portion of the yoke 55 and an armature core 60installed to the outer circumferential portion of the armature shaft 56.And electric power is supplied to this motor 52 via a power supplysection 58 equipped with a brush 61 and a commutator 76 in slidablycontact therewith.

On the other hand, the ball screw mechanism 53 is of a generally knownconstruction in which a large number of balls 64 are intermediatelyinserted between a nut section 62 and a screw section 63. In this case,the nut section 62 is held rotatably in the housing 72 by an angularbearing 65 in the condition housed in a holder 75. The rotating force ofthe motor 52 is transmitted from the armature shaft 56 to the rack shaft51 via the nut section 62, balls 64, and screw section 63 in the form ofaxial reciprocating motion, and assists the steering power.

Now, in the power steering unit of FIG. 5 comprising in this way, bothhousings 71 and 72 are assembled by directly coupling the housings withscrews. In a conventional power steering unit, right and left housingshave a flange protrudably formed at each end portion, respectively, andby connecting these with bolts, the housings are coupled. However, sincethe power steering unit is generally installed near the road surface ofthe car body and must be housed in a limited space below the engineroom, it is not desirable to have protrusions on the outer circumferenceof the device and the improvements have been waited.

Therefore, in the power steering unit of FIG. 5, a system for couplingthe housings with screws as shown in FIG. 6 is adopted, and here, thedevice configuration free of protrusions with the flange removed hasbeen achieved. FIG. 6 is an enlarged sectional view of a main portionshowing the configuration of the coupling portion. In the figure, afemale threaded portion 73 is formed on the left inner circumference ofthe housing 71, while a male threaded portion 74 is formed on the rightend outer circumference of the housing 72. And by driving a malethreaded portion 74 into this female threaded portion 73, both housings71 and 72 are coupled together.

However, in the power steering unit of FIG. 5, since both housings 71and 72 are formed with same material by aluminum die casting, etc.,there is a problem in that the contact surface wears and is easy toadhere by surface pressure and friction caused by tightening.Consequently, lubrication and surface treatment are required betweenboth housings, also resulting in a problem of disadvantages from theviewpoint of man-hour or parts cost.

In addition, because both housings 71 and 72 are formed with nonferrousmetals such as aluminum, etc. for weight reduction, there was a problemof decreased coupling strength or rigidity at the threaded portion.Consequently, as shown in FIG. 6, the wall thickness at the threadedportion must be increased to secure the strength of the couplingportion, also producing a problem of going against needs of reducedweight, reduced thickness, and reduced size.

It is an object of the present invention to prevent wear and adhesionand to reduce the wall thickness with the coupling strength of housingssecured in an electric power steering unit of a configuration in whichhousings are coupled with screws.

The above-described and other objects and novel feature of the presentinvention will become apparent more fully from the description of thefollowing specification in conjunction with the accompanying drawings.

DISCLOSURE OF INVENTION

The electric power steering unit according to the present inventioncomprise a rack- shaft to be coupled to the guiding wheels of a vehicle,a first housing for movably containing the rack-shaft and holdingtherein a coupling section for causing the teeth of the rack-shaft toengage those of a pinion arranged on the steering column linked to thesteering handle of the vehicle, an electric motor equipped with a yokeintegrally formed with the first housing and coaxially arranged aroundthe rack-shaft to supply assisting power to the rack-shaft, a ball screwmechanism for connecting a nut section linked to the armature shaft ofthe electric motor with a screw section formed on the rack-shaft bydisposing ball members therebetween and for transmitting the rotarypower of the electric motor to the rack-shaft as assisting power forsteering, and a second housing for containing the ball screw mechanism,characterized in that a first threaded portion is formed at the endportion of the yoke on the second housing side, a second threadedportion is formed to fit to the first threaded portion on the endportion of the second housing on the first housing side; and the firsthousing and the second housing are coupled together by fitting the firstthreaded portion to the second threaded portion.

With this configuration, the first threaded portion and the secondthreaded portion fitted to it are able to be formed with differentmaterials, and it becomes possible to prevent wear or adhesion on thecontact surface caused by fitting screws of the same material.

On the other hand, the first threaded portion may be formed into femalescrew and the second threaded portion into male screw, and in such case,it is possible to form the female screw on the steel-made yoke,increasing the coupling strength and rigidity at the threaded portion,and this no longer requires to form the coupling portion to have largewall thickness, and the housing is able to reduce wall thickness.

In addition, the first threaded portion may be made into a male screwand the second threaded portion into a female screw. Preferably, thefirst threaded portion may be made of steel, and the second threadedportion may be made of aluminum.

Furthermore, caulking grooves may be formed on the outer circumferenceof the second housing, and the end portion of the first housing on thesecond housing is caulked to these caulking groove, thereby formingcaulking portions to fix the first and the second housings. Preferably,the caulking grooves may be formed continuously throughout the wholecircumference of the second housing or a plurality of caulking groovesmay be intermittently formed on the outer circumference of the secondhousing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view, partly in portion, of the overallconfiguration of an electric power steering unit, which is oneembodiment of the present invention.

FIG. 2 is a sectional view showing the configuration of a main portionof the electric power steering unit of FIG. 1.

FIG. 3 is a sectional view showing the configuration of a main portionof a modification of the electric power steering unit of FIG. 1.

FIG. 4 is an explanatory view showing a configuration of a modificationof a housing coupling portion.

FIG. 5 is an explanatory view, partly in portion, of the overallconfiguration of a conventional electric power steering unit.

FIG. 6 is an enlarged sectional view showing the configuration of a mainportion of the electric power steering unit of FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the invention will be described in greater detail by referring tothe accompanying drawings that illustrate preferred embodiments of theinvention. FIG. 1 is an explanatory view, partly in portion, of theoverall configuration of an electric power steering unit which is oneembodiment of the present invention and FIG. 2 is a sectional viewshowing the configuration of a main portion thereof.

As seen from FIG. 1, the electric power steering unit (hereinafterreferred to as “power steering unit”) of the present invention isprovided with an electric motor 1 having a hollow central area andarranged around a rack-shaft 2 as in the case of the conventional powersteering units shown in FIGS. 5 and 6 and, for example, the unit isfitted to the steering gear of the wheels of a vehicle that aretypically the front wheels. The steering assisting power generated bythe motor 1 is transmitted to the rack-shaft 2 via a ball screwmechanism 3 to reduce the physical load for steering the vehicle on thepart of the driver.

The power steering unit further comprises a housing A (first housing)formed integral with a yoke 7 of the motor 1 and a housing B (secondhousing), which are connected by screws as shown in FIGS. 1 and 2, andthe rack-shaft 2 is arranged in them in such a way that it may be movedto the right and the left direction in FIG. 1. Then, the power steeringunit 1 is fitted to the vehicle main body by means of a bracket (notshown) and fitting holes 10 of the housing A and, at the same time, thewheels are linked to the respective ends of the rack-shaft 2 by means oftie rods or knuckle arms.

On the other hand, the housing A is a hollow member typically made ofaluminum die-casting, to which a cylindrical motor yoke (hereinafterreferred to as yoke) 7 is integrally cast. On the right side end ofhousing A in FIG. 1, a coupling section 6 is arranged for coupling therack-shaft 2 with the steering column 5 of the vehicle that is furtherlinked to the steering wheel. In this coupling section 6, a pinion (notshown) arranged on the steering column 5 and the teeth (not shown) of arack arranged on the outer side portion of the rack-shaft 2 are engagedwith each other so that the rotary motion of the steering column 5 isconverted into a reciprocative motion of the rack-shaft 2 that movesright and left direction in FIG. 1. Note, a torque sensor (not shown) isarranged on the coupling section 6 to detect the torque of the steeringcolumn 5 so that the power output of the motor 1 may be controlled as afunction of the detected torque.

The electric motor 1 has a cylindrical yoke 7 containing coaxiallytherein a cylindrical armature shaft 11 and a field device 12. Therack-shaft 2 is assembled to runs through the inside of the armatureshaft 11. The field device 12 comprises a plurality of magnets 14arranged on the inner peripheral surface of the yoke 7 and an armaturecore 15 arranged on the outer peripheral portion of the armature shaft11 and is fed with electric power from a power supply section 13. Theyoke 7 is a hollow and cylindrical member made of iron and having aprofile of a cylindrical pipe with a substantially constant thickness,and as described above, it is cast into the housing A. A notch (may behole, protrusion, etc.) (not shown) is provided to the yoke 7, and withthis, the yoke is cast into the housing A in the locked state.

The magnets 14 of the field device 12 operates as field poles and arearranged peripherally at regular intervals in the yoke 7. The armatureshaft 11 is held at an end thereof (the right side end in FIG. 1) by abearing 17 fitted to the housing A and provided on the other end (theleft side end in FIG. 1) with a tapered spline, which is engaged with acorresponding spline formed at an end of a nut section 19 of theball-and-screw mechanism 3 so that the rotary motion of the armatureshaft 11 is transmitted to the nut section 19.

The power supply section 13 is designed to feed the armature withelectricity and comprises a commutator 22 rigidly secured to thearmature shaft 11 and a brush 23 held in contact with the peripheralsurface of the commutator 22 to provide electric contact points betweenthem. In this unit, a so-called disk type commutator is used for thecommutator 22, and the brush 23 comes slidably in contact with the rightend face from the axial direction. The brush 23 is held inside a brushholder 41 made of synthetic resin, and pressed against the commutator 22under a predetermined pressure by means of an elastic member (notshown).

The housing B is, like the housing A, a hollow member typically made ofaluminum die-casting and provided in the inside with a ball-and-screwmechanism 3. The ball-and-screw mechanism 3 per se is known andcomprises a nut section 19, a screw section 30 formed on the outerperipheral wall of the rack-shaft 2 and a number of balls 31 arrangedbetween the nut section 19 and the screw section 30. The rack-shaft 2 issupported by the nut section 19 in such a way that its rotary motionaround the axis of rotation is restricted but it is reciprocated rightand left direction in FIG. 1 as the nut section 19 is rotated.

The nut section 19 is axially rotatably held relative to the housing Bwith an angular bearing 4 interposed therebetween and securely fitted tothe housing B. Thus, the nut section 19 of the ball-and-screw mechanism3 and the angular bearing 4 are combined to form a unitary member andarranged in the housing B that is separated from the housing A in thisunit. The angular bearing 4 is securely held between a bearing holderring 32 screwed into an opening of the housing B and a stepped section33 formed in the housing B to restrict its axial movement. The relativeaxial movement of the nut section 19 and the angular bearing 4 isrestricted by another bearing holder ring 34 screwed into the left endof the nut section 19 and another stepped section 35 formed on the outerperipheral wall of the nut section 19.

On the other hand, the housing A and the housing B is coupled by fittingthe threaded portions formed, respectively. In the power steering unitaccording to the present invention, the yoke 7 integrally cast with thehousing A is provided in such a form that it is extended to the housingB side in the axial direction as compared to that of FIG. 5, and afemale threaded portion (first threaded portion) 36 is formed on theinner surface of the left end portion. As against this, on the housing Bside, a male threaded portion (second threaded portion) 37 fitted to thefemale threaded portion 36 is formed on the periphery at the right endportion, and by screwing this male threaded portion 37 into the femalethreaded portion 36, both housings A and B are coupled together. By theway, in this unit, the female threaded portion 36 is formed beforehandin the yoke 7 prior to forming the housing A, and the yoke 7 with thefemale threaded portion 36 formed is integrally cast inside the housingA in such a manner that the yoke 7 is unable to move in thecircumferential direction.

As described above, in this unit, the female threaded portion 36 isformed into the steel-made yoke 7 integrally formed inside the housingA. Consequently, the yoke forms a combination of different materialsfrom the male threaded portion 37 made of aluminum, which is thecounterpart to be coupled, and therefore, it is able to prevent wear andadhering on the contact surface caused by a conventional combination ofsame materials. This eliminates a need for lubricating orsurface-treating the threaded portion, and results in advantage from theviewpoint of cost. In addition, since the female threaded portion 36 ismade of steel, the coupling strength and rigidity of the threadedportion increase, and this also eliminates a need to form the couplingportion with increased wall thickness, and makes it possible to reducewall thickness of the housing A.

Note, after the male threaded portion 37 is screwed into the femalethreaded portion 36, the periphery of the housing A may be caulked atseveral places for locking to tightly fix the housing A to housing B.FIG. 3 is a sectional view of the main portion showing one modificationin which such caulking was carried out. In this case, by caulking theleft end portion of housing A, a caulking portion 39 is formed on thecaulking groove 38 formed continuously or intermittently in plural onthe periphery of the housing B, and both housings A and B are fixed.

On the other hand, the power steering unit operates in a manner asdescribed below. Firstly, as the steering wheel is operated by thedriver to turn the steering column 5 and the rack-shaft 2 is moved inthe direction corresponding to the sense of rotation of the steeringcolumn 5 to carry out a necessary steering operation. When by thisoperation, a steering torque sensor (not shown) is actuated, electricpower is fed to the commutator 22 via the brush 23 in accordance withthis detected torque. As the motor 1 is driven to operate the armatureshaft 11 and hence the nut section 19 coupled to it are rotated. As thenut section 19 rotates, the steering assisting power is transmitted tothe rack-shaft 2 under the effect of the ball-and-screw mechanism 3,whereby the movement of the rack-shaft 2 is promoted and the steeringpower is assisted.

Detailed description has hereinabove been given of the inventionachieved by the present inventor with reference to the embodiment.However, the present invention should not be limited to the embodimentdescribed above, and may be variously modified within the scope notdeparting from the gist.

For example, in the present invention, a housing coupling portion was ofa configuration to couple the female screw section 36 formed in the yoke7 to the male threaded portion 37 formed in the housing B, butconversely, for example, as in the case of FIG. 4, the male threadedportion (first threaded portion) 42 may be formed in the yoke 7, and thefemale threaded portion (second threaded portion) 43 may be formed inhousing B. And in this case also, the male screw section 42 and thefemale screw section 43 may be formed with different materials. Inaddition, the configuration of the present invention may be applied tothe regular general motor, in which a threaded portion is formed in theyoke making the best of the yoke made of steel.

Furthermore, a brushless electric motor may be used for an electricpower steering unit according to the invention. If such is the case, thefield device has a core and magnets respectively arranged on the yokeside and the armature shaft side while the power supply section has acircuit board comprising conducting means and a detector for detectingthe angular position of the armature shaft. Additionally, the presentinvention is applicable not only to front wheels steering unit but alsoto rear wheels steering unit having the same constitution and mountingcondition as the front wheels steering unit.

As described above, according to the present invention, forming thefirst threaded portion on the end portion of the yoke, and fitting it tothe second threaded portion formed on the end portion of the housing Ato couple both housings A and B make it possible to form the first andthe second threaded portions with different materials, and enableprevention of wear and adhesion of the contact surface caused by fittingscrews of same material.

In addition, forming a female screw for the first threaded portion formsthe female threaded portion on the yoke made of steel, which is able toincrease the coupling strength and rigidity of the coupling portion ofthe screw. Consequently, it is no longer necessary to form the couplingportion in increased wall thickness, and it becomes possible to reducewall thickness of the housing, and thereby the whole equipment can bemade lightweight, thinner, and compact.

What is claimed is:
 1. An electric power steering unit for use with avehicle having guiding wheels, a steering handle, a steering columnlinked to the steering handle, and a toothed pinion arranged on thesteering column, said steering unit comprising: a toothed rack-shaft tobe coupled to the guiding wheels of the vehicle, a first housing formovably containing the rack-shaft and holding therein a coupling sectionfor causing teeth of the rack-shaft to engage those of the pinionarranged on the steering column, an electric motor equipped with a yokeintegrally formed with the first housing and coaxially arranged aroundthe rack-shaft to supply assisting power to the rack-shaft, a ball screwmechanism connecting a nut section linked to an armature shaft of theelectric motor with a screw section formed on the rack-shaft bydisposing ball members therebetween and for transmitting the rotarypower of the electric motor to the rack-shaft as assisting power forsteering, and a second housing containing the ball screw mechanism, saidarmature shaft carrying an armature core having first and second endswith the first end being closer to the coupling section than the secondend, the first housing and the yoke both extending fully along thearmature core between the first and second ends of the armature core, afirst threaded portion being formed on an end portion of the yokedefining the farthest extent of the yoke away from the coupling section,a second threaded portion being formed to fit to the first threadedportion on an end portion of the second housing defining the closestapproach of the second housing toward the coupling section, and thefirst housing and the second housing being coupled together by fittingthe first threaded portion to the second threaded portion.
 2. Anelectric power steering unit according to claim 1, wherein the firstthreaded portion is formed into a female screw and the second threadedportion is formed into a male screw.
 3. An electric power steering unitaccording to claim 1, wherein the first threaded portion is formed intoa male screw and the second threaded portion is formed into a femalescrew.
 4. An electric power steering unit according to claim 1, whereinthe first threaded portion is made of steel, and the second threadedportion is made of aluminum.
 5. An electric power steering unitaccording to claim 1, wherein caulking grooves are formed on an outercircumference of the second housing, and the end portion of the firsthousing is caulked to these caulking grooves to fix the first and thesecond housings.
 6. An electric power steering unit according to claim5, the caulking grooves are formed continuously throughout the wholecircumference of the second housing.
 7. An electric power steering unitaccording to claim 5, wherein the caulking grooves are intermittentlyformed in plural on the outer circumference of the second housing.